Asymmetric surface texturing for use in a photovoltaic cell and method of making

Abstract

A novel surface texturing provides improved light-trapping characteristics for photovoltaic cells. The surface is asymmetric and includes shallow slopes at between about 5 and about 30 degrees from horizontal as well as steeper slopes at about 70 degrees or more from horizontal. It is advantageously used as either the front or back surface of a thin semiconductor lamina, for example between about 1 and about 20 microns thick, which comprises at least the base or emitter of a photovoltaic cell. In embodiments of the present invention, the shallow slopes are formed using imprint photolithography.

What is claimed is: 
     
       1. A method to form a photovoltaic cell, the method comprising:
 defining a cleave plane in a monocrystalline semiconductor donor body; 
 affixing a first surface of the semiconductor donor body to a receiver element; 
 cleaving a monocrystalline semiconductor lamina from the semiconductor donor body along the cleave plane, the semiconductor lamina having a thickness between about 0.5 and about 50 microns,
 wherein the semiconductor lamina remains affixed to the receiver element, 
 wherein the cleaving step creates a second surface of the semiconductor lamina substantially parallel to the first surface; and 
 
 treating the second surface, the treating step comprising:
 fabricating a negative template having a negative template surface, the negative template surface having negative template relief; 
 placing the negative template surface adjacent to the second surface, wherein photoresist is disposed between the negative template surface and the second surface; and 
 forming lamina relief on the second surface, wherein the lamina relief is substantially the inverse of the negative template relief, with or without vertical exaggeration or compression. 
 
 
     
     
       2. The method of  claim 1 , wherein the thickness of the semiconductor lamina is between about 0.5 and about 30 microns. 
     
     
       3. The method of  claim 1 , wherein the thickness of the semiconductor lamina is between about 0.5 and about 20 microns. 
     
     
       4. The method of  claim 1  wherein the semiconductor lamina is substantially silicon. 
     
     
       5. The method of  claim 1 , wherein the photovoltaic cell comprises the semiconductor lamina. 
     
     
       6. The method of  claim 1  wherein the semiconductor lamina comprises at least a portion of a base of the photovoltaic cell. 
     
     
       7. The method of  claim 1 , wherein, after the step of treating the second surface, at least 50 percent of the second surface is first faces and second faces, wherein the first faces are at an angle between ten and twenty-five degrees from horizontal, and the second faces are at least seventy degrees from horizontal. 
     
     
       8. The method of  claim 1  wherein the negative template is formed of a transparent or semitransparent material. 
     
     
       9. The method of  claim 1  wherein the step of forming lamina relief on the second surface comprises:
 curing the photoresist while it is disposed between the negative template and the lamina; and 
 etching the photoresist. 
 
     
     
       10. The method of  claim 9  wherein the negative template is formed of a transparent or semitransparent material, and wherein the step of curing the photoresist comprises exposing the photoresist to ultraviolet light. 
     
     
       11. The method of  claim 1  wherein the step of fabricating a negative template having a negative template surface comprises:
 forming a positive template having a positive template surface, the positive template surface having positive relief; 
 providing the negative template, wherein the negative template has a negative template surface, the negative template surface being substantially absent of relief; 
 placing the positive template surface adjacent to the negative template surface with photoresist disposed between the positive template surface and the negative template surface; and 
 forming negative relief on the negative template surface, wherein the negative relief is substantially the inverse of the positive relief, with or without vertical exaggeration or compression. 
 
     
     
       12. The method of  claim 11  wherein the positive template is a monocrystalline silicon wafer. 
     
     
       13. The method of  claim 12  wherein the step of forming the positive template comprises sawing the monocrystalline silicon wafer from an ingot at an angle between about ten and about twenty-five degrees from the {111} crystallographic plane.